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Below-ground interactions at the tree-crop interface in the highlands of Kenya

Govindarajan, Muthiah

Titre : Below-ground interactions at the tree-crop interface in the highlands of Kenya

Auteur : Govindarajan, Muthiah

Université de soutenance : UNIVERSITY OF FLORIDA

Grade : Doctor of Philosophy (PhD) 1996

Competition for growth resources between woody and nonwoody (crop) species is said to be the main reason for crop failure in hedgerow intercropping (alleycropping) in the semiarid tropics. However, the mechanisms of such interspecies competitions have not been clearly understood. Results of a series of investigations on these aspects, conducted in the highlands of Kenya, are reported here. In the first experiment, soil-water changes and root dynamics in a leucaena (Leucaena leucocephala Lam de Wit.) and maize (Zea mays L.) alleycropping system were monitored. Soil water content decreased more rapidly and total root density of leucaena and maize (expressed as cm root-length cm$\sp-3$ soil) increased under the hedgerow intercropping system compared to the sole crop system. Furthermore, the intercropped maize had higher root density than sole-cropped maize i.e., when maize was grown alone. Nutrient contribution through root turnover of the woody species in a cropping season of about 120 days was estimated as 7 kg N and 0.2 kg P ha$\sp-1$. The influence of pruning the above-ground biomass of the woody species on the extent of water availability in the system was examined in another study. Soil water availability was higher under the periodically-pruned hedges than under the unpruned tree. Maize produced more roots when grown under unpruned trees than with hedges or as a sole crop. A third study showed that above-ground pruning of leucaena reduced the number and distribution of its structural roots ; however, when the root system was modified artificially by placing a galvanized-iron-sheet barrier around the tree up to 100 cm soil depth, the number and distribution of structural roots increased. In summary, when grown with a woody component, maize produced more roots than under sole-cropping condition and root production was high when soil water availability was less. Soil water was depleted more rapidly under hedgerow intercropping than under maize-sole-cropping condition. Above-ground pruning seemed to influence the structural root ($>$2 mm in diameter) distribution of the woody species, but its effect on fine-root ($<$2 mm in diameter) density was not clear since root density was also influenced by soil-water status. Future research should focus on the effect of time and frequency of pruning the woody species on its root dynamics and influence on growth of alleycrops.


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